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Last data update: 2014.03.03

R: Generate a weather object

setMet

R Documentation

Generate a weather object

Description

To run Yplant, a weather object needs to be constructed, that contains solar position data, radiation, air temperature, and so on. This function generates a daily diurnal weather dataset using a fairly standard weather generator, or constructs the weather object with user-specified data. See Details.

A built-in weather generator simulates the following variables:

altitude,azimuth: Position of the sun (degrees).
PAR: Photosynthetically active radiation (mu mol m-2 s-1).
fbeam: Fraction direct beam of PAR (-).
Tair: Air temperature (deg C).
VPD: Vapor pressure deficit.

The following two variables are user input, and have no within-day variation:

Ca: Atmospheric CO2 concentration (ppm). (Default = 390ppm).
Patm: Atmospheric pressure (kPa). (Default = 1.01kPa).

If you are curious about the algorithm, please check the code (type setMet).

To generate a weather dataset, simply use this command:

aprilday <- setMet(richmond, nsteps=12, Tmin=9, Tmax=25, month=6, day=21)

Where richmond is a Yplant location object, generated with setLocation.

The weather object can be plotted: the following command produces a simple built-in graph of PAR, Tair, VPD and fbeam:

 plot(aprilday)

Alternatively, the user can input a dataframe (or CSV file) that contains the weather variables (or a subset of them). For example,

 mymet <-
data.frame(Tair=20, PAR0=seq(5,1000,length=10), fbeam=0, Ca=400)

The names of the variables need to be *exactly* as described above (and are case-sensitive!).

If solar altitude and azimuth are not provided, they will be calculated from the location object. In the case that fbeam = 0, though, the solar position has no effect and is ignored, and not calculated.

Usage

setMet(location = NULL, metdat = NULL, year = 2012, month = NA,
  day = NA, nsteps = 10, PARday = 22, AtmTrans = 0.76, fbeamday = NA,
  fbeammethod = c("spitters", "constant"), Tmin = 10, Tmax = 25,
  VPDmax = NA, maxlag = 0.1, Ca = 390, Patm = 101)

Arguments

`location`	A Yplant location object (class 'yplocation', see `setLocation`).
`metdat`	Optionally, a dataframe (or name of CSV file) with standard weather variables.
`year`	Optional (slight effects on solar path).
`month`	1-12
`day`	day of month
`nsteps`	number of steps (will affect number of simulation steps in `YplantDay`.
`PARday`	Total daily PAR on a horizontal surface (MJ m-2 d-1).
`AtmTrans`	Atmospheric transmission.
`fbeamday`	Daily beam fraction.
`fbeammethod`	If 'Spitters', uses the Spitters algorithm to estimate fbeam by timestep, otherwise it is constant (and given by `fbeamday`.
`Tmin, Tmax`	Daily minimum and maximum temperature (deg C).
`VPDmax`	Optional. Daily maximum VPD (if not given, estimated from `Tmin`).
`maxlag`	Lag of temperature maximum behind solar maximum (fraction of day).
`Ca`	Atmospheric CO2 concentration (ppm).
`Patm`	Atmospheric pressure (kPa).

Value

An object of class 'ypmet', a list with the following components:

dat: A dataframe with the weather variables (see Details for a description).
method: Either 'generated' (weather generator was used), or 'input' when user provided metdata.
daylength: in hours
sunset,sunrise: in hours
location: A Yplant location object (class 'yplocation', see setLocation

Author(s)

Remko Duursma. Solar path and diffuse partitioning code borrowed from Maestra (thanks to Belinda Medlyn).

References

For fraction diffuse radiation, uses the 'Spitters algorithm':

Spitters, C.J.T., Toussaint, H.A.J.M., Goudriaan, J., 1986, Separating the diffuse and direct component of global radiation and its implications for modeling canopy photosynthesis. Part I. Components of incoming radiation, Ag. For. Meteorol., 38:217-229.